1. Field of the Invention
The present invention relates to a vacuum circuit breaker which is excellent in high current breaking characteristics, and more particularly, it relates to contact material for the same.
2. Description of the Prior Art
Vacuum circuit breakers, which are maintenance-free, pollution-free and excellent in breaking performance, have been widely used in the art. With development thereof, awaited is provision of circuit breakers applicable to both higher voltage and higher current.
Performance of a vacuum circuit breaker mainly depends on contact material for the same. Such contact material is preferable to have (1) larger breaking capacity, (2) higher withstand voltage, (3) lower contact resistance, (4) smaller force required to separate welded contacts, (5) smaller contact consumption, (6) smaller chopping current, (7) better machinability and (8) sufficient mechanical strength.
It is practically difficult to obtain a contact material having all of the said preferable characteristics. In practical contact material, therefore, only particularly important characteristics required for a specific use are improved at the sacrifice of the other characteristics. For example, a copper (Cu) - tungsten (W) contact material as disclosed in Japanese Patent Laying-Open Gazette No. 78429/1980 is excellent in withstand voltage performance, and thus commonly applied to load switchs, contactors etc. However, the Cu-W contact material is not so much satisfactory in current breaking performance.
On the other hand, a copper (Cu) - chromium (Cr) contact material disclosed in, e.g., Japanese Patent Laying-Open Gazette No. 71375/1979 is remarkably excellent in breaking performance, and thus commonly applied to circuit breakers etc. However, the Cu-Cr contact material is inferior in withstand voltage performance to the Cu-W contact material.
In addition to the aforementioned examples, examples of contact materials generally used in the air or oil are described in literature such as "General Lecture of Powder Metallurgy" edited by Yoshiharu Matsuyama et al. and published (1972) by Nikkan Kogyo Shinbun. However, such contact materials of silver (Ag) - molybdenum (Mo) and Cu-Mo systems as described in "General Lecture of Powder Metallurgy" pp. 229-230 are inferior in withstand voltage performance to the aforementioned Cu-W contact material as well as in current breaking performance to the said Cu-Cr contact material, and thus are scarcely applied to vacuum circuit breakers at present.
As mentioned above, practically selected and employed is a contact material which is excellent in characteristics required for a specific use. However, desired in recent years are vacuum circuit breakers which are applicable to both higher current and higher voltage, and it is difficult to satisfy characteristics required therefor by a conventional contact material. Further, a contact material having higher performance is desired also for miniaturizing the vacuum circuit breakers.